RFR: 8264054: Bad XMM performance on java.lang.MathBench.sqrtDouble

[Sandhya Viswanathan]

For the j.l.Math JMH at https://github.com/openjdk/jmh-jdk-microbenchmarks/blob/master/micros-jdk11/src/main/java/org/openjdk/bench/java/lang/MathBench.java, the performance for sqrt benchmark could be improved.  Thanks a lot to Eric Caspole for finding this issue.

Benchmark:
    @Benchmark
    public double  sqrtDouble() {
        return  Math.sqrt(double4Dot1);
    }

Current code generated (linux format) by c2 JIT is:
     vsqrtsd 0x50(%r10),%xmm0,%xmm0

The vsqrtsd instruction operation is specified as below: 
     VSQRTSD (VEX.128 encoded version)
     DEST[63:0] := SQRT(SRC2[63:0])
     DEST[127:64] := SRC1[127:64]
     DEST[MAXVL-1:128] := 0

The upper 127:64 bits are set from previous contents of xmm0. As the destination xmm0 register was not initialized prior to use by c2 JIT, this causes stall and lower performance.

By adding xmm0 initialization prior to use, the performance of the above benchmark improves significantly.

Code generated after patch:
     vxorpd %xmm0,%xmm0,%xmm0
     vsqrtsd 0x50(%r10),%xmm0,%xmm0

Performance before patch:
Benchmark                    Mode  Cnt       Score      Error   Units
MathBench.sqrtDouble        thrpt    8  193612.396 ±  95.807  ops/ms

Performance after patch:
MathBench.sqrtDouble        thrpt    8  276388.024 ± 846.372  ops/ms

Best Regards,
Sandhya

-------------

Commit messages:
 - No xor needed when src and dest are same
 - 8264054: Bad XMM performance on java.lang.MathBench.sqrtDouble

Changes: https://git.openjdk.java.net/jdk/pull/3256/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3256&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264054
  Stats: 16 lines in 1 file changed: 11 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3256.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3256/head:pull/3256

PR: https://git.openjdk.java.net/jdk/pull/3256